Of the various steps which are required for the production of semiconductor elements, the lithographic step is especially important. Stated most simply, each lithographic step begins by coating a wafer, especially a silicon wafer, with a thin layer of photosensitive material referred to as a photoresist or, simply, a resist, the material of this coating being sensitive to a beam of radiation, for example an ion beam or other particle beam.
A lithographic apparatus projects the beam through a mask provided with a structure to be imaged on the resist, usually in the form of openings in the mask.
Between the mask and wafer, optical elements, i.e. elements capable of modifying the configuration of the beam, can be provided.
The image on the wafer has an extent which is usually far smaller than that of the wafer surface upon which the image is produced. Subsequent to the projection, therefore, the wafer may be shifted and the process repeated so that the same structure of the mask is projected on other locations of the wafer.
The stepping of the wafer through a series of exposures by which the same pattern is imaged on the resist can be repeated until the entire wafer surface coated by the resist is utilized.
A subsequent development of the resist provides the desired pattern on the wafer in the form of resist-free locations. The wafer can then be subjected to any of numerous processes, including etching, ion implantation or coating and diffusion to apply doping elements. After these further steps, the wafer is inspected, recoated with resist and the entire sequence described above is repeated some 8 to 15 times until the result is a checkerboard arrangement of identical microcircuits on the wafer.
Most of the projection lithographic processes used heretofore employ light to irradiate the resist. However, the continuous need for imaging ever smaller structures and the ever higher densities of the components of the microcircuit has mandated the investigation of other radiation methods which are not as limited in resolution as is light with its relatively long wavelength.
Considerable efforts have been made, for example, to attempt to use X-rays in lithographic systems while other processes such as particle-beam lithography and, especially, ion-beam lithography, have been considered as well but with significantly less attention.
In U.S. Pat. No. 4,985,634, an imaging system for lithography purposes is described which comprises a particle source, especially an ion source, a mask provided with a structure to be imaged in the form of one or more components in the mask along the particle or ion beam, means for supporting a wafer in the path of that beam downstream of the mask and, between the mask and the wafer, at least two collecting lenses which are capable of affecting the beam. These collecting lenses are in the form of rotationally symmetrical electrostatic lenses with electrodes of conventional shapes (tubular, diaphragms or combinations thereof) each lens suffering from considerable third-order image distortion which can be only limitedly influenced by the special configuration of the lens geometry. However, by the selection and arrangement of the two lenses this system allowed third-order image distortion to be largely eliminated while a significant fifth-order image distortion, resulting from the product of the aberration coefficient matrix, remained.